The current mainstream of semiconductor devices resides in diodes, transistors, ICs, LSIs and VLSIs of the resin encapsulation type. Epoxy resins have superior moldability, adhesion, electrical properties, mechanical properties, and moisture resistance to other thermosetting resins. It is thus a common practice to encapsulate semiconductor devices with epoxy resin compositions. Semiconductor devices are now used in every area of the modern society, for example, in electric appliances and computers. As a guard against accidental fire, the semiconductor devices are required to be flame retardant.
In semiconductor encapsulating epoxy resin compositions, halogenated epoxy resins combined with antimony trioxide (Sb2O3) are often included in order to enhance flame retardance. This combination of a halogenated epoxy resin with antimony trioxide has great radical-trapping and air-shielding effects in the vapor phase, thus conferring a high fire-retarding effect.
However, the halogenated epoxy resins generate noxious gases during combustion, and antimony trioxide has powder toxicity. Given their negative impact on human health and the environment, it is desirable to entirely exclude these fire retardants from resin compositions.
In view of the above demand, studies have been conducted on the use of hydroxides such as Al(OH)3 and Mg(OH)2 or phosphorus based fire retardants such as red phosphorus and phosphates in place of halogenated epoxy resins and antimony trioxide. Unfortunately, various problems arise from the use of these alternative compounds. The hydroxides such as Al(OH)3 and Mg(OH)2 have less flame retardant effects and must be added in larger amounts in order that epoxy resin compositions be flame retardant. Then the viscosity of these compositions increases to a deleterious level to molding, causing molding defects such as voids and wire flow. On the other hand, the phosphorus-containing fire retardants such as red phosphorus and phosphates added to epoxy resin compositions can be hydrolyzed to generate phosphoric acid when the semiconductor devices are exposed to hot humid conditions. The phosphoric acid generated causes aluminum conductors to be corroded, detracting from reliability.
To solve these problems, Japanese Patent No. 2,843,244 proposes an epoxy resin composition using a flame retardant having red phosphorus coated with a surface layer of SixOy although it still lacks moisture-proof reliability. JP-A 10-259292 discloses an epoxy resin composition in which a cyclic phosphazene compound is used in an amount to give 0.2–3.0% by weight of phosphorus atoms based on the total weight of other constituents excluding the filler. To provide flame retardance, a substantial amount of the cyclic phosphazene compound must be added to the epoxy resin composition, which can cause ineffective cure and a lowering of electrical resistance in a high-temperature environment.